Organic electroluminescent elements (hereinafter, organic electroluminescent elements will be referred to as “organic EL elements”), which are easier to reduce in size, lower in power consumption, and capable of surface light emission, and the use thereof in various display devices such as flat display is attracting attention since the applied voltage can be significantly reduced compared to liquid crystal elements.
Such an organic EL element has a light-emitting layer and an electron-transport layer provided at the cathode side of the light-emitting layer, the electron-transport layer transporting electrons to the light-emitting layer. Examples of the light-emitting layer include blue light-emitting layers that emit blue light, red light-emitting layers that emit red light, and green light-emitting layers that emit green light, and the like.
In particular in an the organic EL element having a blue light-emitting layer, because the blue light-emitting layer demands a greater energy for emitting blue light, holes that do not contribute the light emission in the blue light-emitting layer are transferred easily into the electron-transport layer, compared to an organic EL element having a red or green light-emitting layer. The holes transferred into the electron-transport layer are said to bind with an organic matter contained in the electron-transport layer to generate cations, which degrade the electron-transport layer. Therefore, there is a problem in that the luminance of the blue light emitted by the blue light-emitting layer decreases over time, so that the life of the organic EL element is shortened.
There is another problem in that the holes transferred into the electron-transport layer also recombine with electrons to cause the electron-transport layer to emit light in a color other than blue within a wavelength region close to the peak wavelength of blue, thereby deteriorating the blue chromaticity of the light emitted from the blue light-emitting layer.
Hereinafter, explanation on the chromaticity will be given. FIG. 1 is a diagram showing the chromaticity coordinates in the XYZ color system. The chromaticity is expressed with the values on the two-dimensional chromaticity coordinate (x, y) in the XYZ color system specified by CIE (International Commission on Illumination). The chromaticity (x, y) is obtained from emission spectrum distribution. Ideal red chromaticity point A, ideal blue chromaticity point B, and ideal green chromaticity point C are shown in FIG. 1. Displays are desired to achieve high chromaticity of each of the lights in three primary colors, red, blue, and green, and a greater area (color reproduction range) of the triangle defined by the sides connecting the three chromaticity points. A high chromaticity means that any of the three primary colors, red, blue, and green approaches to its ideal chromaticity point A, B, or C. In the case of blue, the chromaticity is better when the value on y coordinate is closer to 0.
A method of improving the red chromaticity of an organic EL element having a red light-emitting layer has been proposed (for example, see Patent Document 1 below) in which a red light-emitting material is included in the electron-transport layer adjacent to the cathode side of the red light-emitting layer, holes and electrons not contributing to the light emission from the red light-emitting layer recombine at the red light-emitting material to emit light in the same color, red, as the red light-emitting layer.
However, if the method is applied to an organic EL element blue light-emitting layer, there is a problem in that the selection of the electron-transporting material constituting the electron-transport layer is greatly restricted. This is because: a blue light-emitting material that emits blue light has to be contained in the electron-transport layer; but the energy gap of the electron-transport layer has to be greater than that of the blue light-emitting material in order to allow the blue light-emitting material to emit light because the energy gap of the blue light-emitting material is great.
A method of improving the chromaticity of the emission color of a light-emitting layer has been proposed as a conventional technique (for example, see Patent Document 2 below) in which a hole-blocking layer is provided between a light-emitting layer and an electron-transport layer so as to block the holes migrating through the light-emitting layer into the cathode side and so as to restrict emission at the electron-transport layer.
However, because it is difficult to block the holes migrating into the electron-transport layer completely by the hole-blocking layer, there are also problems in that the holes that have migrated through the hole-blocking layer into the electron-transport layer recombine with electrons to cause the electron-transport layer to emit a color other than blue near the peak wavelength of blue, resulting in reduction in the chromaticity in blue—the emission color of the blue light-emitting layer. There are also problems in that the electron-transport layer is deteriorated by the holes transferred into the electron-transport layer, which causes deterioration of the electron transporting performance of the electron-transport layer, gradual decrease in blue luminance over time, and consequent shortening of the life of the organic EL element.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 10-231479    Patent Document 2: Japanese Patent Application No. 10-2986